I don't want to sounds like the grim reaper, but I would be concerned. This sounds just like what happened to my baby Katie Lyn (RIP angel). Hubby and I noticed she was thin and we could feel her ribs, so we brought her in to see the vet. After running tons of tests, she was diagnosed with severe kidney disease.
She was only a littler over 1 year old.
I won;t get into details because it was a very painful time for me and it still is, but I would have your baby checked out. Everyone told me not to worry about it because Katie was naturally a tiny dog -- only 2.5 lns full grown. But thank god I listened and immediately brought her to the vet. There was nothing they could do ... she was in the final stages of it, and it was only a matter of time before it would have killed her (although she did not die from the kidney disease ... she was tragically killed in a car accident). But even had that accident not happened, she only have a few months left.
So please, bring your baby to the vet just to be safe. I'm sure she will be fine but you really never know and it's better to safe then sorry. If you want anymore info on kidney disease, just PM me and I will e-mail you all of the info. I have a lot of research saved (as you can imagine, I researched kidney disease to the fullest once she was diagnosed with it).
Good luck with your baby -- I'm sure she will be fine.
PS - An EXCELLENT test to determine kidney diease (besides a FULL blood panel) is a FRESH stool sample -- meaning you'd have to catch her pee pee in a vile or tube and bring it to the vet immediately (within 24 hours). That's the best test for them to check the protein levels in the urine.